The present invention relates to monitoring of disturbances in the operating parameters of power transmission lines. In particular, the invention concerns the detection and subsequent recording to data descriptive of such disturbances.
In the field of electrical power engineering, generating systems for producing electrical power are interconnected in a complex power grid by high voltage alternating current (AC) three-phase electric power transmission lines. Occasionally, a transmission line is faulted when, for example, a conductor wire breaks and falls to the ground or conductor wires short-circuit together. Other disturbances can occur at the source of the electrical power itself, such as variations in peak voltage or current, frequency changes. Early detection and characterization of a disturbance in an electrical power transmission system is essential to a quick resolution of the problem. Some disturbances can lead to blackouts of a faulted section, while other disturbances cause problems to a power customer who may depend upon receiving electrical power within prescribed operating parameters. It is understood that references to faults or disturbances are intended to encompass any type or nature of abnormality in AC signal or power transmission.
Consequently, disturbance detectors have been developed in which various operating parameters of a power line are compared with preset parameters to determine the character and amount of a deviation. Some detectors have been used with disturbance recorders in which analog representations of the parameters of interest are recorded and displayed.
More recent devices incorporate microprocessor technology to operate on digital representations of the AC power signals. One example is the line disturbance monitor shown in the patent to Bagnall el al., U.S. Pat. No. 4,484,290. Bagnall describes a monitor which receives analog signals from the transmission power line and converts the signals to a digital representation. The monitor includes storage means having a plurality of storage locations, of which a predetermined number are assigned to pre-disturbance operation to store sequentially generated words representative of the sampled AC signal. A remaining number of storage zones are assigned to post-disturbance operation to sequentially store sample words once an AC disturbance has been detected. In the Bagnall device, a disturbance means includes a processor arrangement which receives the AC converted data and compares this data to a number of values indicative of optimal AC operating parameters. Until a disturbance is detected, the pre-disturbance memory storage locations are sequentially overwritten. However, once the Bagnall line disturbance monitor detects a disturbance, the second group of memory locations is accessed for sequential storage of the post-disturbance data.
One difficulty with the Bagnall device is that it requires memory external to the CPU memory of the microprocessor used to perform many of the monitor's functions. Moreover, it does not provide means for storing pre-disturbance data, which data can be important in assessing the cause of a line disturbance. The presently known digital line disturbance devices suffer from these and other defects. For instance, many of these devices are incapable of storing new or old fault data when a second disturbance occurs. In addition, many digital disturbance monitors have no capability of determining the amount of pre- and post-fault data to be stored for subsequent output to various display devices.
Another problem with prior art line disturbance monitors is that the A/D converted signal data is used to perform comparisons for detecting line disturbances or faults. Fault detection by comparing signal data to optimum parameters restricts the type and characteristic of disturbances that can be detected by the monitor. An optimum method of performing the fault or disturbance triggering is to convert the incoming AC signal information to a phasor representation of the signal. This phasor representation can be used to perform a wide variety of fault calculations for comparison to known parameters. Phadke et al. have described a method of obtaining voltage phasors for use in detecting line disturbances which involves a recursive computation for the real and imaginary phasor components. This technique is discussed in "A New Measurement Technique for Tracking Voltage Phasors, Local System Frequency, and Rate of Change Frequency," IEEE Paper No. 82, SM 444-8, A. G. Phadke, J. S. Thorpe, and M. G. Adamiak (1982). In the Phadke et al. approach, a recursive equation is used to determine the phasor representation of the input signal based on digitized signal data. This phasor is subsequently used to calculate AC operating parameters such as phase angle, positive sequence voltage, and line synchronization parameters using a microprocessor-based routine.
Using phasor representations of the AC signal permits the digital line disturbance monitor to rapidly assess many types of fault conditions and line disturbances. However, there still remains a need for a line disturbance monitor that efficiently combines the phasor technique of AC signal representation with high-speed microprocessor technology to more rapidly assess triggering events. There is also a need for a monitoring system that permits user-controlled recording to pre-fault, fault and post-fault data.